A diffraction grating is any arrangement which is equivalent in its action to a number of parallel equi-distant slits of the same width. By studying the intensity patterns produced by electro-magnetic radiation which is incident upon a diffraction grating, the study of various spectra is possible. For this reason the diffraction grating is an extremely powerful tool for divining information from radiant sources.
Generally, in diffraction gratings, the widths of the individual slits are small compared to the wavelengths of electromagnetic radiation which impinge upon the grating. Therefore, when electromagnetic radiation is incident upon the grating, characteristic diffraction patterns are created and may be viewed such as on a screen at some distance from the grating. These diffraction patterns are spectral lines that carry unique information about the source. The diffraction pattern is a series of lines which are a function of the width, d, of the diffraction grating, the wavelength of the light, .lambda., the angle of incidence of the light to the diffraction grating, .theta., and m, the number of orders present in the pattern.
This yields the standard grating equation: EQU d sin .theta.=m.lambda.
which may be generalized to: EQU d(sin i+sin .theta.)=m.lambda.
wherein i is the angle of incidence of the radiation and .theta. is the transmittance angle between the normal and the path of the rays.
While diffraction gratings are very useful in spectroscopy they do not function to produce high order energy fluxes in the interior of the gratings themselves. Matter at temperatures above absolute zero emits electromagnetic radiation over a broad wavelength spectrum. The emitted energy depends not only on the temperature but also upon the material properties, surface conditions, and direction of emission. It is well known that the maximum emissive power is that of a blackbody. The emissive power per unit surface area, E.sub.b, is given by the Stefan-Boltzman Law, while the spectral emissive power, E.sub.b,.lambda., for a blackbody is given by Plancks' Law, provided the wavelength is much less than the characteristic linear dimension of the blackbody. Furthermore, the directional emissive properties of a blackbody obey Lambert's Law such that if a radiometer was moved over the surface of a hemisphere of radius r above a blackbody aperture of elemental area da, the measured radiation intensity would vary as the cosine of the polar angle. Lambert's Law yields the following equation for directional blackbody intensity: ##EQU1## The spectral blackbody intensity is given by: ##EQU2## For many materials the actual directional intensity, I, is obtained by multiplying I.sub.b the directional emissivity yielding: EQU I=.epsilon..sub..theta..phi. I.sub.b
wherein E.sub.O.phi. is the directional emissivity.
The directional spectral intensity for a smooth surface is obtained in the same manner by using the directional spectral emissivity .epsilon..sub..lambda..theta..phi.; EQU I.sub..lambda. =.epsilon..sub..lambda..theta..phi. I.sub.b,.lambda.
Hence, this emissivity is the ratio of the actual emitted intensity to that of a blackbody of the same temperature for the same wavelength and the same direction: ##EQU3## The directional spectral polarized emissivity for the s-polarized electromagnetic field is: ##EQU4## For the polarized electric field the p is substituted for s. The p-polarized electric field is parallel to the plane containing the surface normal and direction of observation. The s-polarized field is perpendicular to the surface normal and direction of observation. The term emittance is used instead of emissivity for surfaces which are not pure materials and/or not smooth.
The study of electromagnetic absorption on diffraction gratings has been studied classically. Examples of such studies may be found in, R. W. Wood, "On a Remarkable Case of Uneven Distribution of Light in the Diffraction Grating Spectrum", Philos. Mag., 4, 396-402 (1902); and C. Harvey Palmer, "Diffraction Grating Anomalies. II. Coarse Gratings", J. Opt. Soc. Am., 46 (1), 50-53 (1956). Prior studies deal with shallow gratings having aspect ratios of less than unity. The aspect ratio is the grating depth, H, divided by the grating repeat distance, .LAMBDA.. The interaction of a p-polarized electromagnetic wave with a diffraction grating gives rise to rapid bright and dark variations in the reflected spectrum which is termed as a "singular anomaly". Singular anomalies are associated with resonant absorption processes on the grating. Furthermore, they correspond with the onset or disappearance of particular spectral diffraction orders. The singular anomalies are known as Rayleigh wavelengths, .lambda..sub.R, and depend on the polar angle and grating repeat distance .LAMBDA. as: EQU .lambda..sub.R =.sub.m.sup..LAMBDA. [sin .theta..+-.1]
where m is an integer. Studies with diffraction gratings having depths greater than the wavelength produced anomalies in the s-polarized light not predicted by earlier theories which assumed H was much greater than the wavelength.
The calculations and measurements for regular surface structures have generally assumed that the radiant wavelength is very small compared to the physical dimension, S, of the surface structure. In this regime, a geometric optical interpretation may be applied. Also, there is no spectral dependence, other than that which arises from the particular material. Comprehensive reviews of measurements with "V-shaped" and other differently shaped grooves are given in the literature, for example, P. Demont, M. Hvetz-Aubert, H. Trann'guyen, "Experiment on Theoretical Studies of the Influence of Surface Conditions on Radiative Properties of Opaque Materials", Int. J. Thermophysics 3, 335-364 (1982).
Furthermore, the geometrical and mathematical theories which explain the s and p-polarized radiant emittances from gratings do not explain the existence of large maxima in s and p-polarized emittance when the repeat distance is comparable to or only slightly less than the depth.
While much is known about diffraction gratings and blackbody radiation, it has not been known to achieve high flux densities internal to certain types of gratings. In particular, the unique properties of microarchitectural deep well surfaces has not been known heretofore. A fortiori, the use of such surfaces has not been known previously.